A side airbag device for a rear seat, that is provided with an airbag that protects from the shoulder portion to the waist portion of a seated passenger, is disclosed in following Patent Document 1. Further, a two-chamber-type side airbag device for a rear seat, that is divided by a seam into a lower inflation chamber (a waist portion chamber) and an upper inflation chamber (a from the shoulder to the chest portion chamber), is disclosed in following Patent Document 2.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-083835    [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-210371